Galaxy morphological classification : Wikis (The Full Wiki) Main article: Hubble sequence The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936. [1] It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented. Hubble’s scheme divides galaxies into 3 broad classes based on their visual appearance (originally on photographic plates ): Tuning-fork style diagram of the Hubble sequence Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge , which is similar in appearance to an elliptical galaxy . They are given the symbol S. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB.

Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies , the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxy, such as irregular galaxies , which have no obvious regular structure (either disk-like or ellipsoidal). The Hubble sequence is often represented in the form of a two-pronged fork, with the ellipticals on the left (with the degree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the two parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy . de Vaucouleurs system NGC 6782 : a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar.

NGC 7793 : a spiral galaxy of type SA(s)d. The Large Magellanic Cloud : a type SBm galaxy. The de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence , first described by Gérard de Vaucouleurs in 1959. [2] de Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies - based on the tightness of the spiral arms and the presence or absence of a bar - did not adequately describe the full range of observed galaxy morphologies. In particular, he argued that rings and lenses were important structural components of spiral galaxies. [3] The de Vaucouleurs system retains Hubble’s basic division of galaxies into ellipticals , lenticulars , spirals and irregulars . To complement Hubble’s scheme, de Vaucouleurs introduced a more elaborate classification system for spiral galaxies, based on three morphological characteristics: Bars. Galaxies are divided on the basis of the presence or absence of a nuclear bar. de Vaucouleurs introduced the notation SA to denote spiral galaxies without bars, complementing Hubble’s use of SB for barred spirals.

He also allowed for an intermediate class, denoted SAB, containing weakly barred spirals. Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not (usually because they are edge-on to the line-of-sight). Rings. Galaxies are divided into those possessing ring-like structures (denoted ‘(r)’) and those without rings (denoted ‘(s)’). So-called ‘transition’ galaxies are given the symbol (rs). Spiral arms. As in Hubble’s original scheme, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms. The de Vaucouleurs scheme extends the arms of Hubble’s tuning fork to include several additional spiral classes: Sd (SBd) - diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central bulge Sm (SBm) - irregular in appearance; no bulge component Im - highly irregular galaxy Most galaxies in these three classes were classified as Irr I in Hubble’s original scheme.

In addition, the Sd class contains some galaxies from Hubble’s Sc class. Galaxies in the classes Sm and Im are termed the “Magellanic” spirals and irregulars, respectively, after the Magellanic Clouds . The Large Magellanic Cloud is of type SBm, while the Small Magellanic Cloud is an irregular (Im). The different elements of the classification scheme are combined - in the order in which they are listed - to give the complete classification of a galaxy. For example, a weakly-barred spiral galaxy with loosely-wound arms and a ring is denoted SAB(r)c. Visually, the de Vaucouleurs system can be represented as a three-dimensional version of Hubble’s tuning fork, with stage (spiralness) on the x-axis, family (barredness) on the y-axis, and variety (ringedness) on the z-axis. [4] Advertisements Numerical Hubble stage de Vaucouleurs also assigned numerical values to each class of galaxy in his scheme.

Values of the numerical Hubble stage T run from -6 to +10, with negative numbers corresponding to early-type galaxies (ellipticals and lenticulars) and positive numbers to late types (spirals and irregulars). Elliptical galaxies are divided into three 'stages': compact ellipticals (cE), normal ellipticals (E) and transition types (E+). Lenticulars are similarly subdivided into early (S-), intermediate (S0) and late (S+) types. Numerical Hubble stage Galaxies and the Universe - Galaxy Classification Galaxy Classification Galaxies show a vast range of forms, and faced with any such situation we would like to seek any underlying patterns. This allows a compact description of individual objects, and if we are fortunate will lead to physical understanding (the prototype system of this kind is the MK stellar classification). Galaxy classification has developed with this aim, from rough description of an image through distinctions among components with different astrophysical properties. It is important to keep in mind that existing schemes are based on the appearance of a galaxy at optical wavelengths (usually in blue light), over the range of surface brightness conveniently recordable from the Earth's surface.

As such, these classifications are dominated by certain components of galaxies. There are crude hand-waving arguments implying that any outer faint structure might not be fundamental dynamically - based on lack of star formation there and the small number of crossing times at such distances - but we certainly need to check, and in fact very deep imaging has shown some surprising structures. The original definition of classification criteria in blue light also has implications for use on galaxies at substantial redshifts, where we typically view in the emitted ultraviolet. Recent surveys in the UV and near-IR have led to the notion of a "morphological K-correction", a shift in stage of the morphological classification due to changes in observed wavelength. This becomes especially important if we want to compare populations of galaxies at very different redshifts, so that different sets of galaxies are seen at different emitted wavelengths. As a specific example of how galaxies can change in appearance, here is a multiwavelength series of images of M81 = NGC 3031. Some of the same effects can be seen by comparing (observed) optical and near-infrared structures of faint galaxies, such as this example from WFPC2 and NICMOS imaging in the Hubble Deep Field .

These are, left to right: a ROSAT image, from data archived at HEASARC; a color-mapped UV image from GALEX; an optical color image, from data provided by G. Bothun; a near-IR image from the 2MASS atlas [Atlas Image obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF], and a 70-μ image from Spitzer. Comparing these images, starlight from the general population is important from the UV to the near-IR. Only recent star formation is important in the mid-UV, with the bulge vanishing. A more subtle shift occurs between optical and near-IR. Much of the bulge light can now be associated with a broad barlike oval distortion, and the spiral arms are now seen mostly by the light of red giants - not only are they smoother, but the ridge lines of the arms don't quite match those traced by young stars in the opti cal and UV.

These young tracers tell where the density wave has driven cloud collapse, which may have a delay from the deepest potential perturbation and may not match the smoothed potential traced by the older stars. Thought question: What kinds of galaxy classifications would have arisen if we had first encountered galaxies via the UV, or in the near-infrared? How about observing from downtown Rio or the surface of Pluto? The earliest schemes were purely descriptive, dating to times before galaxies and gaseous nebulae could be distinguished. An example is Wolf's system from 1908, shown in Fig 1 of Sandage in SSS9 : Already sequences of disk systems with changes in inclination were recognized; Curtis used photographs of such systems to study the distribution of absorbing matter in the disks of spirals even before their nature was known. The first overall scheme (which remains in widespread use) was that of Hubble. This system distinguishes ellipticals, spirals with and without bars, and irregulars. The classification criteria are: Apparent shape of ellipticals. Bulge strength and arm characteristics for spirals (arm characteristics through clumpiness or resolution most important). Spiral arms can sometimes be defined as much by dust as by blue stars.

Hubble types can be conveniently arranged in the famous "Tuning Fork" diagram. The schematic example is taken from Realm of the Nebulae, as reprinted by Sandage 1975 . Ray White has posted a tuning-fork diagram using DSS images . Here is a version including some additional deeper CCD data I had lying around: and one uising images from the Hubble telecope itself: ` The "definitive" version of the Hubble classification was set out in Sandage's article in Galaxies and the Universe. Compared to Hubble's original conception, this version adds the S0 (lenticular) class between ellipticals and spirals. Hubble hypothesized such an intermediate class, but it was only recognized later. Galaxies are often called early (E and S0) or late (Sb,Sc, Irr) in type, a remnant of early notions that galaxies physically evolve along the Hubble sequence. Unfortunately, this nomenclature is opposite to that of the dominant stellar population in these types, and to the early-late nomenclature in the Yerkes classification. Classification of an elliptical galaxy image is straightforward, because there is so little structure present.

Types E0-E7 are recognized, where the number gives the projected axial ratio. Specifically, an E0 galaxy appears circular (like M87), and in general for axial ratio b/a the number is 10(1-b/a). True ellipticals do not appear more flattened than E7, probably because there is a stability limit for nonrotating systems near this shape. Note that even the ellipticity is not completely well-defined, as many ellipticals have changes in ellipticity with radius or isophotal twists. Most appear to be triaxial systems. Spirals are divided into ordinary (S) and barred (SB) families, with a stage from S(B)a ("early" types, large bulge, tightly wound and fairly smooth arms) to S(B)c ("late" spiral, small bulge, loosely would arms, arms very textured with star-forming regions). As noted in the Hubble atlas, some spirals have arms patterns defined more by the dust lanes than by starlight per se. NGC 253 is a prominent example. For much of its extent, I find the foreground galaxy in NGC 3314 to be a personal favorite of this kind.

This classification has gained such currency because (1) it is simple and thus accomodates the great majority of bright galaxies and (2) it is our great fortune that it correlates well with astrophysical quantities like bulge/disk ratio, gas content, star-forming properties, spectrum, chemical composition of ISM, etc. This is especially nice since the classification was originally designed solely to describe galaxies' appearance on photographs! Several refinements toward a continuous sequence have been made. Sandage not only explicitly recognized transition S0 galaxies, but found subtypes depending on the amount of dust in the disk plane (and strength of the disk) denoted S0-, S0,S0+ as in the Hubble Atlas. S0 systems pose some interesting problems of recognition when not seen close to edge-on, since they are tough to tell from ellipticals with extended envelopes. de Vaucouleurs introduced several important features: (1) continuity of barred and nonbarred spirals (through mixed types SA-SAB-SB) (2) continuity of spirals into irregulars Sc-Scd-Sd-Sdm-Sm-Im (m means Magellanic, the LMC being the prototype).

(3) description of the spiral structure is paramount, rather than some resolution estimate. This makes classification less sensitive to an observer's allowance for seeing or equipment changes. He introduced classes depending on whether the inner structure is ringlike or purely spiral, with mixed types recognized: (r),(rs),(sr),(s). The stage along the sequence still runs a-c (adding, in fact, stages d and m - for Magellanic types - out beyond Sc) as in the original Hubble class. For tabulation in the Reference Catalogs and convenient numerical use, a type T is often used which is -1 for S0, 0 for S0/a, on through 5 for Sc and 9 for Im. More negative values denotes various subsets of ellipticals with or without envelopes. The article by de Vaucouleurs in Handbuch der Physik (1959, 53, 275) sets out his detailed classification principles with photographic examples. (4) the notion of a ``classification volume" rather than a one dimensional sequence. The classification volume is depicted in this sketch, most recently reproduced by Combes and Buta (1996, Fund. Cosmic Phys.

17, 95) and posted by permission of Ron Buta: The artist's conception below shows how the SA/SB and r/s classes interact at stage Sb (from Fig.1 of the RC1, likewise scanned in Combes and Buta (1996) and reposted courtesy of Ron Buta). A typical classification in the de Vaucouleurs system looks like SAB(rs)bc (in fact, this is the alleged type of our own galaxy). (5) Recognition of outer ring (prepended R) and pseudoring - denoted by initial (R) - structures. van den Bergh recognized an additional family of smooth-armed ("anemic") spirals, considering now that there exist parallel sequences S0,A,S. Thus the tuning fork becomes a trident (see Fig. 5-8 of Mihalas and Binney). These are most common in pretty rich clusters, but don't seem to be numerous for later Hubble types (small bulge:disk ratios), and the question immediately presents itself as to whether anemic spirals are waypoints between "normal" spirals and S0 systems.

He has also introduced the luminosity class, ranging from I-V, which is based on the degree of order in the spiral pattern. Sc I galaxies have very long organized arms, Sc Vs have very poorly organized patterns. These classes do correlate with absolute magnitude, although not as tightly as first hoped. One still occasionally reads of Sc I spirals as the brightest such objects, used as rough standard candles. Even with the luminosity class from arm structure, the appearance of a spiral is not a reliable guide to its linear size (something that David Block stressed years ago in an atlas published at Fort Hare, South Africa). Pretty similar-looking spirals can often differ in size by a factor 5 - M81 is a pretty small spiral compared to some of the ones in Virgo, and NGC 309, UGC 2885, NGC 3312, and NGC 6872 are huge (the largest extending to observed diameters more than 250 kpc for H0=50). van den Bergh has described many classification issues, morphological and quantitative, in Galaxy Morphology and Classification (Cambridge, 1998).

The Yerkes (or Morgan) scheme uses strictly the relative prominence of disk and bulge (the Hubble classification includes arm structure) - these are generally correlated, but not uniquely. This classification uses a form factor E,S,B, or D (for symmetric but non-E or S systems) and inclination class 1-7 (7 most elongated) plus a spectroscopic type corresponding to the nearest stellar equivalent to the spectroscopic appearance of a typical galaxy of similar morphological structure (confused yet?). This ranges from a-k, so Yerkes types look like Egk1, Iaf3, Sfg7, Bg4. The spectroscopic class is based on visual inspection of the image even though it looks like a spectral type. The system was defined and illustrated in PASP 70,364 (1958) , ApJ 135,1 (1962), ApJ 142, 1364 (1965), and AJ 76,1000.

The types most used from this system are some of the outliers: N-galaxies whose light is dominated by an unresolved nucleus, and cD galaxies for supergiant (spectroscopic nomenclature c) galaxies with extended envelopes. As noted in van den Bergh's book, the concentration changes little over the Hubble type range E-S0-Sa, and changes a great deal within the Sc class, graphically demonstrating that physical properties for any kind of purely morphological classification may have a highly nonlinear mapping to that classification's criteria, even when we are fortunate enough for there to be a useful mapping at all. Classifications of this kind, based on light concentration, have received renewed interest in the context of high-redshift galaxies, and with the recognition that they can be made robust even for poorly resolved systems by appropriate modelling. Vorontsov-Velyaminov produced a purely descriptive scheme that incorporates peculiarities such as three-armed spirals and gamma-forms. This accomodates most of the oddballs that the Hubble scheme doesn't, but does not yet allow a continuous sequence. It was illustrated in vol. 1 of the MCG.

This points up various attitudes toward galaxies that don't really fit in systems like the Hubble classification - is it right to force them into one of the bins? How peculiar is peculiar? As transient phenomena, should interaction-induced distortions be dignified with a place in the scheme et all? (De Vaucouleurs wrote several times that we don't have a special model name for the prodicts of an automotiove collision. But then cars aren't self-gravitating systems). Spiral structure has been divided into grand-design and flocculent types, depending on the level of organization. The Elmegreens (1982 MNRAS 201, 1021; 1987, ApJ 314, 3) use an index that distinguished these, still on a purely visual level. The arm class ranges from 1 ("chaotic, fragmented, unsymmetric spiral arms") to 12 ("two long symmetric arms dominating the optical disk") Such distinctions have been long noted (for example in the Hubble Atlas and in part in van den Bergh's luminosity classes), but interest in disk dynamics eventually provoked a separate numerical estimate of arm organization.

Some galaxies fall outside the commonly recognized sequences. Many of these are "train wrecks", transient forms produced by the interaction or merger of galaxies. These forms include ring (not ringed) galaxies, polar rings, shells, and systems with tails, as well as double nuclei and highly asymmetric galaxies. Possibly related are what Hubble called Irr II and de Vaucouleurs calls I0 systems. The type example is M82; a galaxy with early-type stellar spectrum, but no particular spiral structure and amorphous appearance. These are almost always found in dense environments and appear to result from interaction-induced bursts of star formation. There have been numerous papers purporting to give a ``true" physical explanation for the Hubble sequence. Their authors are, to my mind, missing the point. Classification must begin as a descriptive process; the desired physical insight is a rather different thing. Basic references on classification: for the Hubble system, see the Hubble Atlas, Revised Shapley Ames Catalog, Sandage-Bedke NASA atlas, and Carnegie Atlas of Galaxies.

Luminosity classification of spirals was described by van den Bergh in ApJ 131, 215 (1960) and ApJ 131, 558 (1960); the NASA atlas includes many examples. de Vaucouleurs (1977, Yale conference p. 43) discusses comparison of classification with quantitative parameters. Dwarf galaxies were not included in these original schemes; see Sandage and Binggeli 1984 (AJ 89, 919). Many useful references are given in van den Bergh's review (JRAS Canada 69, 57, 1975) and in Sandage, SSS vol 9. Dwarfs can be classified based on whether a distinct nucleus is present and on whether the structure is spheroidal or more irregular. Some fall in the low-surface-brightness (LSB) category, though this also includes very large and massive systems; our understanding of LSB galaxies and how they relate to other kinds is still in its infancy.

Classification on these systems is fairly robust; experienced observers generally agree on a galaxy's Hubble type to a level DT = 0.7 on the scale where the range from, say, Sa to Sab is one unit (and automated systems can be trained to a similar level). So what is this system "telling" us? The Hubble type (or T) can be shown to correlate with: bulge/disk luminosity ratio relative H I content M(H I)/L(B) mass concentration Hubble Sequence : definition of Hubble Sequence and synonyms of Hubble Sequence (English) Ellipticals The giant elliptical galaxy ESO 325-G004. On the left (in the sense that the sequence is usually drawn) lie the ellipticals . Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer representing their degree of ellipticity on the sky. By convention, is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as for an ellipse with semi-major and semi-minor axes of lengths and respectively.

[5] The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle). Observationally, the most flattened elliptical galaxies have ellipticities e=0.7 (denoted E7). This is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles. See also: Elliptical galaxy Spirals The Pinwheel Galaxy (Messier 101/NGC 5457): a spiral galaxy classified as type Scd on the Hubble sequence The barred spiral galaxy NGC 1300 : a type SBbc On the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies . A spiral galaxy consists of a flattened disk , with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge .

Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, given the symbol SB. Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows: Sa (SBa) - tightly-wound, smooth arms; large, bright central bulge Sb (SBb) - less tightly-wound spiral arms than Sa (SBa); somewhat fainter bulge Sc (SBc) - loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge Hubble originally described three classes of spiral galaxy.

This was extended by de Vaucouleurs [6] to include a fourth class: Sd (SBd) - very loosely-wound, fragmentary arms; most of the luminosity is in the arms and not the bulge Although strictly part of the de Vaucouleurs system of classification, the Sd class is often included in the Hubble sequence. The basic spiral types can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending 2 lower-case letters to the main galaxy type (for example56 Sbc for a galaxy that is intermediate between an Sb and an Sc). Our own Milky Way is generally classed as SBb, making it a barred spiral with well-defined arms. However, this classification is somewhat uncertain since we can only infer how our galaxy would appear to an outside observer. See also: Spiral galaxy Lenticulars The Spindle Galaxy (NGC 5866), a lenticular galaxy with a prominent dust lane in the constellation of Draco .

At the centre of the Hubble tuning fork, where the two spiral arms meet the elliptical branch lies an intermediate class of galaxies known as lenticulars and given the symbol S0. These galaxies consist of a bright central bulge , similar in appearance to an elliptical galaxy , surrounded by an extended, disk -like structure. Unlike spiral galaxies , the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. The bulge component is often the dominant source of light in a lenticular galaxy. [7] Face-on lenticulars are difficult to distinguish from ellipticals of type E0, making the classification of many such galaxies uncertain. When viewed edge-on, prominent dust-lanes are sometimes visible in absorption against the light of stars in the disk. At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly-flattened ellipticals and spirals. Later observations (by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by Allan Sandage .

[8] Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies . The Large Magellanic Cloud (LMC) - a dwarf irregular galaxy Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed irregular galaxies . Hubble defined two classes of irregular galaxy: [9] Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the Magellanic Clouds - two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure [10] in the Large Magellanic Cloud led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the Small Magellanic Cloud (denoted Im).

In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork. See also: Irregular galaxy Physical significance Elliptical and lenticular galaxies are commonly referred to together as “early-type” galaxies, while spirals and irregular galaxies are referred to as “late types”. This nomenclature is the source of the common, [11] but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from elliptical galaxies through lenticulars to either barred or regular spirals . In fact, Hubble was clear from the beginning that no such interpretation was implied: The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations are made at one's peril. The entire classification is purely empirical and without prejudice to theories of evolution... [3] The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young stars and regions of active star formation , while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe appears to be dominated by spiral and irregular galaxies.

In the currently favored picture of galaxy formation , present-day ellipticals formed as a result of mergers between these earlier building blocks. Lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation. Shortcomings A common criticism of the Hubble scheme is that the criteria for assigning galaxies to classes are subjective, leading to different observers assigning galaxies to different classes (although experienced observers usually agree to within less than a single Hubble type [12] ). The different classification criteria can also be at odds with each other: for example, a more dominant bulge component does not always go hand-in-hand with more loosely-wound spiral arms. Another criticism of the Hubble classification scheme is that, being based on the appearance of a galaxy in a two-dimensional image, the classes are only indirectly related to the true physical properties of galaxies. In particular, problems arise because of orientation effects (the same galaxy looks very different when viewed edge-on, as opposed to face-on), because visual classifications are less reliable for faint or distant galaxies, and because the appearance of galaxies changes depending on the wavelength of light in which they are observed.

Nevertheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates. [13] See also ^ Longair, M. S. (1998). Galaxy Formation. New York: Springer. ISBN 3-540-63785-0 . ^ de Vaucouleurs, G. ; Oemler, Augustus, Jr.; Butcher, Harvey R.; Gunn, James E. (1955). "Studies of Magellanic Clouds. I. Dimensions and structure of the Large Cloud". The Astronomical Journal 160: 126–140. Bibcode 1955AJ.....60..126D . DOI : 10.1086/107173 . ^ Baldry, I. K. (2008). "Hubble's Galaxy Nomenclature". Astronomy & Geophysics 49 (5): 5.25.

Bibcode 2008A&G....49e..25B . DOI : 10.1111/j.1468-4004.2008.49525.x . ^ Dressler, A.; Oemler, A., Jr.; Butcher, H. R.; Gunn, J.E. (July 1994). "The morphology of distant cluster galaxies. 1: HST observations of CL 0939+4713". The Astrophysical Journal 430 (1): 107–120. Bibcode 1994ApJ...430..107D . DOI : 10.1086/174386 . ^ Roberts, M. S.; Haynes, M. P. (1994). "Physical Parameters along the Hubble Sequence". Annual Reviews of Astronomy & Astrophysics 32 (1): 115–152. Bibcode 1994ARA&A..32..115R .

DOI : 10.1146/annurev.aa.32.090194.000555 . External links Hubble sequence notes - Hubblesequencenotes The Hubble sequence is a Hubble sequence notes Hubble sequence notes - Hubblesequencenotes The Hubble... SCHOOL View Full Document Hubble sequence notes The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented. Hubble’s scheme divides regular galaxies into 3 broad classes - ellipticals , lenticulars and spirals - based on their visual appearance (originally on photographic plates ). A fourth class contains galaxies with an irregular appearance. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy Elliptical On the left (in the sense that the sequence is usually drawn) lie the ellipticals . Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images.

They are denoted by the letter E, followed by an integer representing their degree of ellipticity on the sky. By convention, is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as for an ellipse with semi-major and semi-minor axes of lengths and respectively. [5] The ellipticity increases from left to right on the Hubble diagram, with near- circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle). Observationally, the most flattened elliptical galaxies have ellipticities e=0.7 (denoted E7). This is consistent with their being truly This preview has intentionally blurred sections. Sign up to view the full version. This is the end of the preview. Sign up to access the rest of the document.

TERM Chapter 6 Quiz Review Supernova Ohio Northern University a surface explosion on a white dwarf, caused by falling matter from the atmosphere of Chapter 6 Quiz Review Supernova Classification of Galaxies long version Classification of Galaxies Our galaxy is one of billions of galaxies populating the universe. It would be the height of presumption to think that we are the only living things in that enormous immensity -- Werner Von Braun Become familiar with the Hubble classification system. Become familiar with the similarities and differences between types of galaxies. Look at the differences between the modern and early universe. Introduction In the early part of the 20th century, Edwin Hubble began studying “spiral nebulae.” Distance measurements using the period-luminosity relationship for Cepheid variables would eventually show these “nebulae” were in fact other galaxies outside our own. Hubble came to recognize that there are several types of galaxies based on their visual appearance (morphology). He expanded his studies to include all types of galaxies, and was eventually able to show that the universe is expanding (Hubble’s law). To help his studies, he also developed a morphological classification system for galaxies.

Despite modifications and refinements over the years reflecting our increasing understanding of these beautiful stellar systems, the basic Hubble classification still remains in widespread use today. In Hubble’s classification, galaxies are divided into 5 basic groups: elliptical, lenticular or S0, spiral, barred spiral, and irregular. Elliptical galaxies generlly lack any overal structure except for their general shape. Spirals and barred spirals have a distinct disk and bulge, and get their name from the fact that there appear to be spiral arms in the disk. Lenticulars appear very similar to the ellipticals but actually have a distinct bulge and disk. In most cases, the disk is very hard to make out in optical images. Irregular is the catch-all for everything else.

Each group is further divided based on characteristics within the group: Ellipticals are numbered 0  7 based on their “ellipticity” e (note this is 10 times the eccentricity) e=10(major axis  minor axis)/major axis An E0 would be round, an E7 cigar shaped Spirals and barred spirals are labeled a b or c based on a combination of the relative size of the bulge and wrap of the arms a’s have large bulges and tightly wrapped arms c’s have small bulges and loosely wrapped arms Barred spirals have a round or oval bulge with a nearly rectangular bar across it. The spiral arms attach to the bar rather than the bulge. An Sa would be a galaxy with a large bulge and tightly wrapped arms and an SBc would have a small, oval bulge with arms that attach to the end of the bar and spread out away from the center. Lenticulars generally appear round like an E0 but have a distinct bulge and/or a disk like a spiral. Irregulars are not easily classified into any of these groups. Many irregulars appear to be interacting with other galaxies. In addition, there are peculiar galaxies.

These are classifiable as a particular type but have something strange about them. For example, it may be classifiable as an edge-on type Sc but the spiral disk may be warped, or as a E4 but have a distinct blob of gas near one edge. The first atlas of peculiar galaxies was compiled by Halton C. Arp in 1966. In many cases he classified galaxies as peculiar because there was an unresolved companion, so it looked like there was a blob on the side of the galaxy. Overal there are 16 different classes for galaxies in this system. It works fairly well for the nearby bright galaxies Hubble was studying. However, as astronomy developed, especially at non-optical wavelengths, the need for more catagories arose. Several other classification systems were developed, but the most popualr was developed by Gerard de Vaucouleurs in 1959. He basically expanded the Hubble classification system allowing for inermediate classes such as Sab, and adding extra notations such as + and - signs or extra letters like m to indicate things like the the strength of the bar, disk structure in lenticulars, rings in sprirals, and different structures in irregulars.

Using this classification system, there are hudreds of different types of galaxies possible. This is the system generally used if the Hubble classification system in insufficient. In this activity, you use the Hubble classification system to develop a method for classifying galaxies from a set of pre-classified galaxies. Then you'll classify several mystery galaxies. After that, you will draw some conclusions about the history of several types of galaxies based on the color images. If you want more information on the objects or on any of the catalogues and classification systems, is an excellent place to start. Part 1:The Classification System You need the checklist and 9 images with known classification. Spread the images out in front of you in the same order as the checklist. Based on your overall view of these images, answer the questions below. How do you tell the difference between the dense bulge at the center of a lenticular and a photographic/ printing effect making the center of an elliptical galaxy look like a dense center? How do you determine where the edge of the galaxy is in order to measure its diameter (consider especially the E0 and SBb galaxies)? How do you tell if a galaxy has a disk if it is (a) face on and (b) edge on?

Now you are going to develop a checklist to help you classify galaxies. Answer the following questions for each of the pictures. If the answer is yes, place a check in the appropriate space on the checklist and leave it blank if the answer is no. If it asks for a number, fill in the number in the appropriate space. This section will determine the basic type: elliptical, lenticular, spiral, barred spiral or irregular. Is there a distinct bulge at the center? Is the bulge round? Is there a distinct disk? Are there spiral arms visible? Do the spiral arms attach directly to the bulge? (the other option would be attaching to a bar that cuts across the bulge) If there are spiral arms, rank the wrap of the arms on a scale of 1 to 10 where 1 is very tightly wrapped (circles back around the galaxy) and 10 is very loose (curves away in an arc) Is there any structure other than a bulge visible or anything unusual looking? This section will determine the number or letter for the elliptical or spiral. You can skip these questions if it is an irregular or lenticular galaxy. Measure the major axis of the galaxy.

If it appears to be a round elliptical, take the longest of several measurements. If it is a spiral-type and appears round, take the average of several measurements. Enter this as x in the checklist. Measure the minor axis if it is an elliptical (perpendicular to the direction you measured in step 8), or the diameter of the bulge if it is a spiral or barred spiral (parallel to the direction you measured in step 8). Enter this under the y column in the checklist. Calculate the ellipticity e if it is an elliptical, or the bulge  to  disk b/d ratio for a spiral. Enter this in the last column. Once you have completed the table, you will have a check list to determine the galaxy type based on the known galaxies. Answer the following questions before going onto part 2. What would the checklist look like for an E6 galaxy? What would the checklist look like for an SBa peculiar galaxy? How would you classify a galaxy with loosely wrapped arms but a large round bulge? Why? Part 2: Unknown galaxies Your GSI will give you a list for the unknown galaxies.

You will need to find the images of the galaxies on your list, then determine their classification as outlined below. Begin by recording their names in table 1. In the next column, record which checklist items from numbers 1 - 5 and 7 you answered yes to for each picture, and the rank for 6 if it is a spiral galaxy. If it is an elliptical or spiral galaxy, measure x and y and calculate e or b/d (checklist items 8 - 10), and record the values in the next 3 columns. Any additional comments should go in the second to last column. This is a good place to make notes about how you made your final determination of the galaxy type, especially for peculiars and boarder-line galaxies (e.g. an elliptical with e=4.5 or a spiral with a large bulge and loose arms) Finally, determine the classification and record it in the last column. Questions What problems did you run into classifying the galaxies? How confident are you in your classification: do you think your classification would match if you look up your galaxies in a catalogue? Explain.

Part 3: Colors and what they tell us Join with another group who had a different set of galaxy images and sort your galaxy images into piles according to the basic types. Look at your spiral and barred spiral galaxies. What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that. Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where? Look at the images of the elliptical galaxies What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that. Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where? Look at the images of the lenticular galaxies What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that. Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where?

Look at the images of the irregular galaxies What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that. Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where? Pull out all the images of galaxies you classified as peculiar. What type(s) do the peculiars fall into (elliptical, lenticular or spiral/barred spiral)? Is one type more common? Which one? Is there anything all peculiars of the same basic type have in common that is not common to the other galaxies of the same basic type? What is it? If you have peculiars of more than one basic type, list commonalities for all types. Based on your answers to the above 10 questions, answer the following: Which type(s) of galaxy tends to have old stars? How did you determine this? Which type(s) of galaxies tend to have ongoing star formation? How did you determine this? What sort of event can trigger galaxy-wide star formation? Explain how you determined this event is a trigger.

Part 4: The Hubble UDF Look at the Hubble Ultra Deep Field (UDF) image segment (available here if you want to see the original) Note there are no stars, only galaxies in this image. Light from most of the galaxies in this image left when the universe was about 1 billion years old, so they provide the earliest snapshot yet of what galaxies were like when the universe was young. Estimate the number of galaxies in the image. Describe how you estimated the number. Estimate the number of galaxies you can classify as spiral, elliptical or lenticular. What fraction of the total number of galaxies is this? Estimate the number of irregular galaxies. What fraction of the total number of galaxies is this? Is the Hubble classification system useful for the UDF? How were galaxies in the early universe different from galaxies in the modern universe? Last modified: 7/12/05

Question: What systems are classified based on their visual appearance by the Hubble sequence?
Answer: Galaxies